Jadis PA 100 amp - Jadis becomes Jadis again

[microstrip]

A while back I found what is purported to be a Defy 7 schematic (I’m not sure how to put it here). I just looked at it, and it shows three tubes in parallel for each 1/2 push-pull, with a bias voltage going to each half. So that would make sense with the 4 pots in your photo … one per triplet per channel. I don’t know what the 5th pot is for; the schematic doesn’t show the biasing circuit itself.

I would expect a general common pot for the four other pots. If the tubes are all matched - a Jadis practice, see how obsessed they are with burning and matching tubes in these old great posts of Gary Koh, a Defy 7 enthusiast https://www.whatsbestforum.com/threads/a-visit-to-jadis.10696/ - just one adjustment would be needed, for example, for mains voltage adaptation.

To me that means you should have matched triplets. On the paper you posted, those looked like Gm (transconductance), and maybe plate current? The Gm any tester should be able to check. The plate current, if that’s what it is, would need a specific circuit condition. But I saw on one of the Upscale videos that he does AR matching differently “because they run them hot”, so maybe he could actually set up a specific test. (Not trying to sell him, just info for you, FWIW).

It does make you wonder, though, why they didn’t just do a trim pot for each tube, so at least the operating condition could always be made “safe”.

In push pull amplifiers with common bias we need matched plate current (IP) at the operating bias current - usual meters do not have such capability. I built a simple jig to measure them at 65mA - the operating conditions of ARC and cj amplifiers. Now the VTL measures them for me!

Most vendors match power tubes at 30 or 35mA at around 500V plate voltage - for ARC it should be carried at an higher current 65mA and lower plate voltage - around 430V, if I remember well. BTW, using two voltmeters and a good life insurance we can easily measure tubes for matching in an amplifier. Just measure the bias grid voltage individually (remove other tubes) at the correct cathode current and than match for equal grid voltage. In the Defy 7 we can measure 4 tubes each time!

 

[jadis]

I would expect a general common pot for the four other pots. If the tubes are all matched - a Jadis practice, see how obsessed they are with burning and matching tubes in these old great posts of Gary Koh, a Defy 7 enthusiast https://www.whatsbestforum.com/threads/a-visit-to-jadis.10696/ - just one adjustment would be needed, for example, for mains voltage adaptation.



In push pull amplifiers with common bias we need matched plate current (IP) at the operating bias current - usual meters do not have such capability. I built a simple jig to measure them at 65mA - the operating conditions of ARC and cj amplifiers. Now the VTL measures them for me!

Most vendors match power tubes at 30 or 35mA at around 500V plate voltage - for ARC it should be carried at an higher current 65mA and lower plate voltage - around 430V, if I remember well. BTW, using two voltmeters and a good life insurance we can easily measure tubes for matching in an amplifier. Just measure the bias grid voltage individually (remove other tubes) at the correct cathode current and than match for equal grid voltage. In the Defy 7 we can measure 4 tubes each time!

Nice info, Microstrip


While I love tubes , been using them since 1985, I am not ready to die for them.

 

[jadis]

And I got info from a good friend who also has the PA100 and wants to buy new tubes, he read from a net source that the 5th trim pot is for the input driver tubes' bias adjustment, first time I've ever heard of a such an adjustment but of course I'm not an expert.

 

[sel53]

Phil,
I had another thought … If you have a channel that you think is working within spec, you could measure the voltage across the resistor connected to the plate (pin 3) or cathode (pin 8) (assuming there is one), and bias the other side to match (on the average). Measuring across a cathode resistor would be much lower voltage, if that is a concern. You also may be able to just measure from cathode to ground.

That wouldn’t change the benefits of having matched tubes, but it at least might get you out of the red-tube problem.

I hope you don’t mind these random thoughts, I imagine this is causing you some distress!

 

[jadis]

Phil,
I had another thought … If you have a channel that you think is working within spec, you could measure the voltage across the resistor connected to the plate (pin 3) or cathode (pin 8) (assuming there is one), and bias the other side to match (on the average). Measuring across a cathode resistor would be much lower voltage, if that is a concern. You also may be able to just measure from cathode to ground.

That wouldn’t change the benefits of having matched tubes, but it at least might get you out of the red-tube problem.

I hope you don’t mind these random thoughts, I imagine this is causing you some distress!

SEL,

Your thoughts are much appreciated. But if the method you suggest involves opening the bottom cover and lying the amp on its side while checking the voltages, I'm afraid I am not equipped to do that. Yes, it is stressful now.

 

[AudioLibertarian]

SEL,

Your thoughts are much appreciated. But if the method you suggest involves opening the bottom cover and lying the amp on its side while checking the voltages, I'm afraid I am not equipped to do that. Yes, it is stressful now.

You should only let the qualified tech do it, there are some live wires there!

 

[microstrip]

And I got info from a good friend who also has the PA100 and wants to buy new tubes, he read from a net source that the 5th trim pot is for the input driver tubes' bias adjustment, first time I've ever heard of a such an adjustment but of course I'm not an expert.

Interesting, also never read about such thing. The old Audio Research D70 had an adjustment for balance of the input stage - I remember we had to adjust it for minimal second harmonic distortion. At that time I had to built a distortion meter according to an Hifi News and Record Review DIY project - there were no soundcard audio spectrometers around!

 

[jadis]

Interesting, also never read about such thing. The old Audio Research D70 had an adjustment for balance of the input stage - I remember we had to adjust it for minimal second harmonic distortion. At that time I had to built a distortion meter according to an Hifi News and Record Review DIY project - there were no soundcard audio spectrometers around!

Interesting indeed.

 

[jadis]

I have now decided to order from the local dealer 12 Jadis selected EL34 tubes as he will have his techman install and bias them in my house. The cost is 1/3 that of the KT88s. Jadis will then give him the instructions. Someone in a Jadis user group said that this info is 'secret'.

 

[AudioLibertarian]

I have now decided to order from the local dealer 12 Jadis selected EL34 tubes as he will have his techman install and bias them in my house. The cost is 1/3 that of the KT88s. Jadis will then give him the instructions. Someone in a Jadis user group said that this info is 'secret'.

It will sound better as well with the EL 34 ! Less power thou than the Kt 120, yet much more refined, totally worth imho

 

[jadis]

It will sound better as well with the EL 34 !

Would the EL34s still give out the same 150w output power?

 

[AudioLibertarian]

I doubt it, but it depends on the voltage plate supplied to tube.... I would think about 120-125 watts per channel,but you can have your tech measure power output with the proper gear ....

 

[jadis]

I doubt it, but it depends on the voltage plate supplied to tube.... I would think about 120-125 watts per channel,but you can have your tech measure power output with the proper gear ....

Got a message from the local dealer that Jadis said the total power output with the EL34 tubes is the same at 150w. That is also what is mentioned in the Italian Jadis site as the latest production comes in EL34, KT88, and KT 120 with the same power rating of 150w. Now I'm quite eager to have them installed, will take a few weeks for the order to come in.

 

[jadis]

And not surprising, my order of the new fleet of EL34s have been canceled indefinitely due to the lack of tube shipment from Russia. And good thing the local dealer sent me his tech man, equipped with the bias figure and instruction to bias the amp which now has 4 new EH tubes which arrived weeks ago, to blend with the 8 original older Jadis stock KT88s. It's a temporary solution while I now focus on getting a new fleet of Psvane China KT88s from a local dealer. He needs to check the plate voltage of the amp first before he will sell me the Psvane tubes.

Btw, the local Jadis tech man told me that in order to use the EL34s, there is a need to change some resistors. That's new to me.

 

[AudioLibertarian]

And not surprising, my order of the new fleet of EL34s have been canceled indefinitely due to the lack of tube shipment from Russia. And good thing the local dealer sent me his tech man, equipped with the bias figure and instruction to bias the amp which now has 4 new EH tubes which arrived weeks ago, to blend with the 8 original older Jadis stock KT88s. It's a temporary solution while I now focus on getting a new fleet of Psvane China KT88s from a local dealer. He needs to check the plate voltage of the amp first before he will sell me the Psvane tubes.

Btw, the local Jadis tech man told me that in order to use the EL34s, there is a need to change some resistors. That's new to me.

Its a known fact that resistors need to be chnanged to switch to new tubes for Jadis , that is why it is HIGHLY recommended to order the EL 34 version from the factory at the time of purchase, to avoid hassles with tech, etc.

 

[jadis]

And watching the tech man do the bias, it is something that I have done before with my friend's ARC Ref 110. Open cover and work with live voltages with bare sight of the circuit boards. The only difference is that the ARC amps installed black and yellow + and - trim pots beside each power tube (user friendly) and the blue trim pots are clearly near them. The bar copper rods of the Jadis can look menacing but I was told it has 4-5vDc along its path, and it is connected to the 3 fuse holders for the set of 3 tubes to be biased. And the is the + point of test while the - goes to the chassis as ground. The herculian task is really to lift the 120 lbs amp and lay it on its side and then back down after the biasing. The lethal part I guess is accidentally touching the transformer wires which are now facing up instead of ARC's that is facing down and a bit secure.

 

[milesdavis1943]

And watching the tech man do the bias, it is something that I have done before with my friend's ARC Ref 110. Open cover and work with live voltages with bare sight of the circuit boards. The only difference is that the ARC amps installed black and yellow + and - trim pots beside each power tube (user friendly) and the blue trim pots are clearly near them. The bar copper rods of the Jadis can look menacing but I was told it has 4-5vDc along its path, and it is connected to the 3 fuse holders for the set of 3 tubes to be biased. And the is the + point of test while the - goes to the chassis as ground. The herculian task is really to lift the 120 lbs amp and lay it on its side and then back down after the biasing. The lethal part I guess is accidentally touching the transformer wires which are now facing up instead of ARC's that is facing down and a bit secure.

JADIS Amps Are Overrated, Overpriced And Not Worth The Money

 

[jadis]

JADIS Amps Are Overrated, Overpriced And Not Worth The Money

Still, I have to live with it.

 

[jadis]

In the light of the vanishing Russian tubes, the alternative is China PsVane tubes.

 

[sel53]

In the light of the vanishing Russian tubes, the alternative is China PsVane tubes.

Phil,

Phil,
How do the Chinese tubes sound? You always have the remaining originals if you need to go back to that approach!

Good luck!